Magnetorheological (MR) fluids consist of dispersions of magnetically soft particles in a liquid. Particles of the magnetically dispersed phase are magnetically soft in order to allow for reversibility of the magnetic effect. At zero magnetic field, the viscosity of the base MR fluid may be on the order of 0.1-0.7 Pa.cndot.sec and the fluid exhibits ideal Newtonian behavior, i.e. shear stress is directly proportional to shear rate. However, under a magnetic field, a substantial increase in yield stress can occur. The applied field induces a dipole moment in each particle causing the formation of pearl-like chains which form in the direction of the magnetic field. The substantial increase in yield stress is both rapid (within milliseconds) and nearly reversible. This fibril formation is responsible for the observable shear stresses which allow MR fluids to be used for applications such as vibrational dampers, clutches, brake systems, shock absorbers and variable resistance apparatuses.
Magnetorheological (MR) fluids and electrorheological (ER) fluids were originally discovered in the late 1940's. However, research on MR fluids ceased while development of their electric analog continued. This was probably because the large particle sizes of the magnetically active phase of MR fluids led to a strong tendency for the particles to settle out of the liquid phase. Recently, there has been a renewed interest in MR fluids, probably due to the fact that MR fluids can obtain yield stresses approximately two magnitudes larger than ER fluids (.about.100 kPa for MR fluids compared to .about.3 kPa for ER fluids). MR fluids hold many other advantages over ER fluids advantages as well. MR fluids have a high tolerance of common impurities such as water, stability over a wide temperature range (-40 to 150.degree. C.), and the ability to use low voltage power supplies (12 volts). ER fluids, on the other hand, are relatively less tolerant of impurities, which means strict control of processing is required. Furthermore, the need for bulky, high voltage power supplies (5,000 volts) associated with ER fluids can pose a number of design and safety problems. The ER fluids also often lose their strength as their temperature increases.
There are some significant problems associated with conventional MR fluids. One disadvantage is that dispersed particles in MR fluids settle down as a result of gravitational or centrifugal sedimentation. Another related problem is that the settled particles form a tightly knit sediment or a "cake" which, once formed, makes it extremely difficult to redisperse the MR fluid. These problems arise since iron powders (density of approximately 7.8 g/cm.sup.3) and ceramic ferrite powders (density of approximately 5.24 g/cm.sup.3) are denser in comparison with the carrier liquids (densities approximately 0.8-1.0 g/cm.sup.3).
MR fluids originally based on relatively large coarse iron particles (e.g., greater than 50-100 microns) were unsuitable for practical application since the particles would settle out of the liquid phase. Recently, there has been renewed interest in the applications of MR fluids. One of the possible reasons for this is the lack of success in development of the electrical counterpart ER fluids. Also, certain specific applications such as high torque rotary couplings could only be possible with MR fluids.
Researchers have reported magnetic or ferrofluids based on nano-sized magnetic particles having diameters of less than 30 nm. When particle sizes reach 5 to 10 nm, the dispersion acts as a ferrofluid where there is no observable yield stress but rather the entire sample undergoes a body force proportional to the magnetic field gradient. Ferrofluids, behave like liquid magnets and are distinguishable from MR fluids. Ferrofluids are currently being utilized as hermetic seals.
U.S. Pat. No. 5,505,880 to Kormann et al. discloses the use of sodium salt of polyacrylic acid of molecular weight 4,000 and water, along with ethylene glycol or other liquids as a carrier, to prepare MR fluids based on manganese zinc ferrite and other magnetic particles of size less than 1 micron. These types of fluids, although relatively stable against settling, have two undesired characteristics. The first is the yield stress of these fluids is relatively low (approximately maximum 6 kPa). The lack of adequate yield stress will mean that these MR fluids will not be useful for several applications. Furthermore, since the magnetic particles used are ultrafine, the temperature dependence of the yield stress for these fluids is very significant, which poses challenges in designing devices. Furthermore, these fluids contain water and a complicated process is needed to remove the water if anhydrous MR fluids are desired. The presence of water is often a disadvantage because of corrosion problems. Although stable suspensions may be obtained using these fluids, there is an overall decrease in yield stress (maximum yield stress of about 6 kPa).
Another approach disclosed in U.S. Pat. No. 5,354,488 to Shtarkman et al. uses a non-magnetic carbon dispersant, not greater than 10 nm in size, added to MR fluids to enhance their stability. Shtarkman et al. have referred to these materials as electrorheological magnetic (ERM) fluids. Other dispersants such as boron, aluminum, non-magnetic iron, silicon, germanium, and carbides, nitrides, oxides of aluminum, boron, germanium, hafnium, iron, silicon, tantalum, tungsten, yttrium and zirconium, as well as silicon and siloxane organic polymers, non-silicon containing organic polymers, silica-siloxane polymers and mixtures thereof are also disclosed. The MR fluids prepared using such dispersants are stated to be useful in avoiding the so-called "stick-slip" behavior demonstrated by other MR fluids in which the magnetic phase and the carrier fluids separate out, once the magnetic field is applied. In the process disclosed in U.S. Pat. No. 5,354,488, magnetic particles are first mixed with those of the dispersant, and then the carrier fluid is added to prepare the MR fluid. The dispersant particles are stated to be reversibly bound to the magnetic particles by van der Waals forces. The preferred volume of the dispersant was 1 to 7 volume percent based on the volume of magnetic particles. The overall volume of the carrier fluid was about 45 percent. The use of carbon black as a dispersant for MR fluids is also disclosed in U.S. Pat. No. 4,687,596 to Borduz et al.
In U.S. Pat. No. 5,167,850 to Shtarkman, the difference between a dispersant, as discussed above, and a surfactant is disclosed. Surfactants such as ferrous oleates, ferrous napthalates, aluminum tristearates, lithium stearates, sodium stearates, oleic acid, petroleum sulfonates and phosphate esters, almost all of which have been described in the prior art concerning the so-called ferrofluids, could be used in combination with a carbon dispersant for preparing MR fluids.
U.S. Pat. No. 5,398,917 to Carlson et al. and U.S. Pat. No. 5,645,752 to Weiss et al. disclose surfactants and dispersants similar to those discussed above, although no specific examples of dispersants are included. In addition, U.S. Pat. No. 5,398,917 also mentions that particle settling in MR fluids can be minimized by the addition of silica, and that the silica will form a thixotropic network that helps reduce settling of particles. U.S. Pat. No. 5,398,917 also notes that other low molecular weight hydrogen bonding molecules such as water, and other molecules containing hydroxyl, carbonyl, or amine functionality can be used to assist the formation of a thixotropic network. Thus, the low molecular weight agents could consist of water, methyl, ethyl, propyl, isopropyl, butyl, and hexyl alcohols, ethylene glycols, diethylene glycol, propylene glycol, glycerols, amino alcohols and amino esters from 1-16 carbon atoms in the molecule, several types of silicone oligomers, and mixtures thereof. In the examples of U.S. Pat. No. 5,398,917, stearic acid is used as a surfactant and no mention is made of the use of any dispersant. The use of silica as a dispersant for magnetic recording materials as gamma iron oxide has also been previously described.
U.S. Pat. No. 5,578,238 to Weiss et al. discloses the cleaning of surfaces of magnetic particles using chemical or physical processes. This patent also discusses the use of plastics, metals or ceramics to protect the surfaces from corrosion. Examples of metallic materials used to modify the particles surface include titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, copper, silver, gold, lead, tin, zinc, cadmium, cobalt-based intermetallic alloys and nickel-based intermetallic alloys. Examples of plastics used to protect the magnetic materials surface include acrylics, cellulosics, polyphenylene sulfides, polyquinoxilines and polybenzimidazoles. The goal of deposition of such coatings is to protect the magnetic particles from corrosion. This, for example, may be important in magnetorheological fluids containing water.
Each of the above-referenced patents is incorporated herein by reference.
The present invention has been developed in view of the foregoing and to address other deficiencies of the prior art.